1. Field of the Invention
The present invention relates to a display apparatus, which uses ferroelectric liquid crystal (FLC), and a method of driving the same, and, more particularly, to a liquid crystal display apparatus which displays image gradation by a matrix drive method and a method of driving the same.
2. Related Background Art
As for the display apparatus, which uses a ferroelectric liquid crystal (FLC), there has been a known device disclosed in Japanese Patent Application Laid-Open No. 61-94023 and constituted in such a manner that ferroelectric liquid crystal is injected into a liquid crystal cell formed by placing two glass plates, each of which has a transparent electrode formed thereon and whcih have been subjected to an orienting process in such a manner that the two glass plates are placed while having a cell gap of about 1 .mu.m.about.3 .mu.m.
The aforesaid display apparatus which uses ferroelectric liquid crystal has two characteristics. That is, a fact, that the ferroelectric liquid crystal has a spontaneous polarization, causes combining force of an external electric field and the spontaneous polarization to be utilized to be utilized in switching. Another effect can be obtained in that the switching operation can be performed by the polarity of an external electrode because the longer axes of ferroelectric liquid crystal molecules correspond to the directions of the spontaneous polarizations.
The longer axes of the liquid crystal molecule of the ferroelectric liquid crystal are oriented in twisted directions under a bulk condition because the ferroelectric liquid crystal ordinarily uses chiral smectic liquid crystal (SmC* SmH*). However, the aforesaid problem that the longer axes of the lqiuid crystal molecules are undesirably twisted can be overcome by injecting the ferroelectric liquid crystal into the aforesaid cell having the cell gap of 1 .mu.m.about.3 .mu.m. The aforesaid phenomenon has been disclosed in p213 to p234, N. A. CLARK et al., MCLC, 1983, Vol 94 and so forth.
Although the ferroelectric liquid crystal has been mainly utilized as a binary (light and dark) display device having two stable states composed of a light transmissive state and a light shielded state, multi-value images, that is, half tone images can also be displayed. The half tone image display methods are exemplified by a method which realizes a half-tone type light transmissive state by controlling the area ratio in a bi-stable state (the light transmissive state or the light shielded state) in a pixel. Then, the gradation expressing method (hereinafter called an "area modulation method") will now be described.
FIG. 9 is a graph which schematically illustrates the relationship between switching pulse V of the ferroelectric liquid crystal device and transmissive light quantity I of the same, where transmissive light quantity I realized after a single pulse of either polarity is applied to a pixel in an initial state in which it is completely shielded from light (dark state) is plotted as the function of voltage V of the single pulse. If the pulse voltage V is lower than threshold V.sub.th. (V&lt;V.sub.th), the transmitted light quantity is not changed, and the transmissive state after the pulse has been applied is, as shown in FIG. 10B, the same as that shown in FIG. 10A. If the pulse voltage V is higher than the threshold, (V.sub.th &lt;V), a portion in the pixel is brought to another stable state, that is, a light transmissive state as shown in FIG. 10C so that the overall light quantity becomes an intermediate quantity. If the pulse voltage is raised to a value higher than saturation value V.sub.sat (V.sub.sat &lt;V), the overall portion of the pixel is brought into a light transmissive state as shown in FIG. 10D, and therefore the light quantity reaches a predetermined value (saturated).
That is, the area gradation method is a method for forming half tone images corresponding to the applied voltage V by performing a control in which the pulse voltage V is caused to meet V.sub.th &lt;V&lt;V.sub.sat.
However, the following problem arises if the aforesaid simple driving method is employed. That is, the fact that the relationship between the voltage and the transmissive light quantity depends upon the thickness of the cell and the temperature will arise a problem in that a different gradation is displayed depending upon the position in the display panel although a pulse voltage of a predetermined level is applied if a cell-thickness or the temperature is dispersed in the display panel.
FIG. 11 is a graph which illustrates the aforesaid fact, where the relationship between the pulse voltage V and the transmissive light quantity I is shown similarly to FIG. 9. In FIG. 11, the relationship between the two factors at different temperatures, that is, curve H indicating the relationship held at high temperature and curve L indicating the relationship held at low temperature are shown. In general, a display of a type having a large size frequently encounters a fact that the temperatures are dispersed in the same panel. Therefore, when a half tone image is formed at a certain driving voltage V.sub.ap, a problem arises in that the half tone level is distributed irregularly in a range from I.sub.1 to I.sub.2 in the same panel as shown in FIG. 11 and therefore a uniform gradation image cannot be formed.
In order to overcome the aforesaid problem, a driving method (hereinafter called a "4-pulse method") has been disclosed in Japanese Patent Application No. 2-94384 by the applicant of the present invention (inventor: Okada). As shown in FIGS. 8 and 12, the "4-pulse method" is a method in which a plurality of pulses (pulses A, B, C and D shown in FIG. 12) are applied to all of a plurality of pixels positioned on the same scanning line in one panel and having different thresholds so as to obtain the same quantity of transmissive light as shown in FIG. 8.
However, use of the aforesaid "4-pulse method" will arise the following problem in that optical responses of the pixel with respect to the applied writing pulses (A), (B), (C) and (D) are respectively affected by other pulses previously applied to the aforesaid pixel during a process in which the reset pulse (A) is applied to the pixel on a selected scanning line and then gradation information writing pulses (B), (C) and (D) are applied as shown in FIGS. 8 and 12. That is, the voltage (threshold), at which the liquid crystal is inverted, is changed when the next pulse is applied. The aforesaid phenomenon will raise a problem at the time of setting the voltage of the pulse (B). Although the error is included by an allowable range (although the accuracy in expressing the gradation deteriorates) if the influence of the other pulse is limited and the degree of the threshold change is also limited, forming of gradation images cannot be performed by the 4-pulse method if the threshold is changed considerably. The reason for this lies in that the aforesaid "4-pulse method" disclosed in Japanese Patent Application No. 3-73127 is a driving method based on a fact that the inversion characteristics of liquid crystal with respect to the voltages of the four pulses applied to the pixel are the same.
Furthermore, domain walls such as i, j and k (the boundary between the oriented region corresponding to the light state and the oriented region corresponding to the dark region) shown in FIG. 8 must be included by the pixel in the case where the other pulses (B), (C) and (D) are applied because bright and dark domains present in the pixel, to which the voltage has been applied, while being mixed with each other (in a state where a half tone image is displayed) although the pulse (A) shown in FIG. 8 can be set to a voltage level sufficiently higher than the threshold because it is a reset pulse. As described above, the positions of the domain walls i, j and k are affected considerably by the voltage pulse applied immediately as well as the writing pulses (B), (C) and (D) in the case where switching is performed with the voltage which extremely approximates the inversion threshold of the liquid crystal. Although the influence of the other pulse applied immediately before the writing pulses are applied does not raise a critical problem in the case where the change of the voltage of the pulses applied immediately is limited, a problem sometimes arises in that the "4-pulse method" drive cannot be performed if the change has been made considerably.
The aforesaid problem taken place in that the displayed gradation image is undesirably affected by the pulse except for the writing pulses also arises by the other pulse immediately after the writing pulse has been applied. In a case where a domain wall is formed by the pulse (C) at the position j shown in FIG. 8, the domain wall can be sometimes translated if the pulse (for example, a voltage pulse due to an information signal at the time of no selection) following the pulse (C) has a certain voltage level. That is, there is a problem in that the displayed gradation image determined by the writing pulses can be easily subjected to a cross talk which takes place due to the influence of the ensuring pulses.
There arises another problem in that writing takes a too long time in addition to the aforesaid problems of the threshold level change and the cross talk. The reason for this lies in that the "4-pulse method" must use four pulses (A), (B), (C) and (D) in comparison to the conventional driving method in which two pulses are used to write one pixel. As a result, the time (the frame time) required to write image information on the entire surface of the panel is lengthened, causing the quality of a displayed kinetic image to deteriorate. If the worst comes to the worst, kinetic images cannot be displayed.
As described above, the "4-pulse method" encounters a problem of the error taken place when a gradation image is formed or another problem of an unsatisfactory display speed.